Routing Metrics Used for Path Calculation in

Low-Power and Lossy Networks

Abstract

Low-Power and Lossy Networks (LLNs) have unique characteristics
compared with traditional wired and ad hoc networks that require the
specification of new routing metrics and constraints. By contrast,
with typical Interior Gateway Protocol (IGP) routing metrics using
hop counts or link metrics, this document specifies a set of link and
node routing metrics and constraints suitable to LLNs to be used by
the Routing Protocol for Low-Power and Lossy Networks (RPL).

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6551.

Copyright Notice

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

1. Introduction

This document makes use of the terminology defined in [ROLL-TERMS].

Low-power and Lossy Networks (LLNs) have specific routing
characteristics compared with traditional wired or ad hoc networks
that have been spelled out in [RFC5548], [RFC5673], [RFC5826], and
[RFC5867].

Historically, IGP, such as OSPF ([RFC2328]) and IS-IS ([RFC1195]),
has used quantitative static link metrics. Other mechanisms, such as
Multiprotocol Label Switching (MPLS) Traffic Engineering (TE) (see
[RFC2702] and [RFC3209]), make use of other link attributes such as
the available reserved bandwidth (dynamic) or link affinities (most
of the time static) to compute constrained shortest paths for Traffic
Engineering Label Switched Paths (TE LSPs).

This document specifies routing metrics and constraints to be used in
path calculation by the Routing Protocol for Low-Power and Lossy
Networks (RPL) specified in [RFC6550].

One of the prime objectives of this document is to define a flexible
mechanism for the advertisement of routing metrics and constraints
used by RPL. Some RPL implementations may elect to adopt an
extremely simple approach based on the use of a single metric with no
constraint, whereas other implementations may use a larger set of
link and node routing metrics and constraints. This specification
provides a high degree of flexibility and a set of routing metrics
and constraints. New routing metrics and constraints could be
defined in the future, as needed.

The metrics and constraints defined in this document are carried in
objects that are OPTIONAL from the point of view of a RPL
implementation. This means that implementations are free to include
different subsets of the functions (metric, constraint) defined in
this document. Specific sets of metrics/constraints and other
optional RPL parameters for use in key environments will be specified
as compliance profiles in applicability profile documents produced by
the ROLL working group. Note that RPL can even make use of no
metric, for example, using the Objective Function defined in
[RFC6552].

The Destination-Oriented Directed Acyclic Graph (DODAG) root, as
defined in [RFC6550], may advertise a routing constraint used as a
"filter" to prune links and nodes that do not satisfy specific
properties. For example, it may be required for a path only to
traverse nodes that are mains-powered or links that have at least
a minimum reliability or a specific "color" reflecting a user-
defined link characteristic (e.g., the link layer supports
encryption).

A routing metric is a quantitative value that is used to evaluate
the path cost. Link and node metrics are usually (but not always)
additive.

The best path is the path that satisfies all supplied constraints (if
any) and that has the lowest cost with respect to some specified
metrics. It is also called the shortest constrained path (in the
presence of constraints).

Routing metrics may be categorized according to the following
characteristics:

Link versus node metrics

Qualitative versus quantitative

Dynamic versus static

Routing requirements documents (see [RFC5673], [RFC5826], [RFC5548],
and [RFC5867]) observe that it must be possible to take into account
a variety of node constraints/metrics during path computation.

Some link or node characteristics (e.g., link reliability, remaining
energy on the node) may be used by RPL either as routing constraints
or as metrics (or sometimes both). For example, the path may be
computed to avoid links that do not provide a sufficient level of
reliability (use as a constraint) or as the path offering most links
with a specified reliability level (use as a metric). This document
provides the flexibility to use link and node characteristics as
constraints and/or metrics.

The use of link and node routing metrics and constraints is not
exclusive (e.g., it is possible to advertise a "hop count" both as a
metric to optimize the computed path and as a constraint (e.g., "Path
should not exceed n hops")).

Links in LLN commonly have rapidly changing node and link
characteristics; thus, routing metrics must be dynamic and techniques
must be used to smooth out the dynamicity of these metrics so as to
avoid routing oscillations. For instance, in addition to the dynamic
nature of some links (e.g., wireless but also Power Line
Communication (PLC) links), nodes' resources, such as residual
energy, are changing continuously and may have to be taken into
account during the path computation.

It must be noted that the use of dynamic metrics is not new and has
been experimented in ARPANET 2 (see [Zinky1989]). The use of dynamic
metrics is not trivial and great care must be given to the use of
dynamic metrics since it may lead to potential routing instabilities.
That being said, a lot of experience has been gained over the years
on the use of dynamic routing metrics, which have been deployed in a
number of (non-IP) networks.

Very careful attention must be given to the pace at which routing
metrics and attributes values change in order to preserve routing
stability. When using a dynamic routing metric, a RPL implementation
should make use of a multi-threshold scheme rather than fine granular
metric updates reflecting each individual change to avoid spurious
and unnecessary routing changes.

The requirements on reporting frequency may differ among metrics;
thus, different reporting rates may be used for each metric.

The set of routing metrics and constraints used by a RPL deployment
is signaled along the DAG that is built according to the Objective
Function (rules governing how to build a DAG) and the routing metrics
and constraints are advertised in the DODAG Information Object (DIO)
message specified in [RFC6550]. RPL may be used to build DAGs with
different characteristics. For example, it may be desirable to build
a DAG with the goal to maximize reliability by using the link
reliability metric to compute the "best" path. Another example might
be to use the energy node characteristic (e.g., mains-powered versus
battery-operated) as a node constraint when building the DAG so as to
avoid battery-powered nodes in the DAG while optimizing the link
throughput.

The specification of Objective Functions used to compute the DAG
built by RPL is out of the scope of this document. This document
defines routing metrics and constraints that are decoupled from the
Objective Function. So a generic Objective Function could, for
example, specify the rules to select the best parents in the DAG, the
number of backup parents, etc., and it could be used with any routing
metrics and/or constraints such as the ones specified in this
document.

Some metrics are either aggregated or recorded. An aggregated metric
is adjusted as the DIO message travels along the DAG. For example,
if the metric is the number of hops, each node updates the path cost
that reflects the number of traversed hops along the DAG. By
contrast, for a recorded metric, each node adds a sub-object
reflecting the local valuation of the metric. For example, it might
be desirable to record the link quality level along a path. In this
case, each visited node adds a sub-object recording the local link
quality level. In order to limit the number of sub-objects, the use
of a counter may be desirable (e.g., record the number of links with
a certain link quality level), thus, compressing the information to
reduce the message length. Upon receiving the DIO message from a set
of parents, a node might decide, according to the OF and local
policy, which node to choose as a parent based on the maximum number
of links with a specific link reliability level, for example.

Note that the routing metrics and constraints specified in this
document are not specific to any particular link layer. An internal
API between the Medium Access Control (MAC) layer and RPL may be used
to accurately reflect the metrics values of the link (wireless,
wired, PLC).

Since a set of metrics and constraints will be used for links and
nodes in a LLN, it is critical to ensure the use of consistent metric
calculation mechanisms for all links and nodes in the network,
similar to the case of inter-domain IP routing.

There are many different permutations of options that may be
appropriate in different deployments. Implementations must clearly
state which options they include, and they must state which are
default and which are configurable as options within the
implementation. Applicability statements will be developed within
the ROLL working group to clarify which options are applicable to the
specific deployment scenarios indicated by [RFC5673], [RFC5826],
[RFC5548], and [RFC5867].

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].

2. Object Formats

2.1. DAG Metric Container Format

Routing metrics and constraints are carried within the DAG Metric
Container object defined in [RFC6550]. Should multiple metrics
and/or constraints be present in the DAG Metric Container, their use
to determine the "best" path can be defined by an Objective Function.

The Routing Metric/Constraint objects represent a metric or a
constraint of a particular type. They may appear in any order in the
DAG Metric Container (specified in [RFC6550]). They have a common
format consisting of one or more bytes with a common header.

The object body carries one or more sub-objects defined later in this
document. Note that an object may carry a TLV, which may itself
comprise other TLVs. A TLV carried within a TLV is called a TLV in
this specification.

Length (8 bits): this field defines the length of the object body,
expressed in bytes. It ranges from 0 to 255.

Res Flags field (16 bits). The Flag field of the Routing Metric/
Constraint object is managed by IANA. Unassigned bits are considered
as reserved. They MUST be set to zero on transmission and MUST be
ignored on receipt.

The following bits of the Routing Metric/Constraint Flag field object
are currently defined:

'P' flag: the P field is only used for recorded metrics. When
cleared, all nodes along the path successfully recorded the
corresponding metric. When set, this indicates that one or
several nodes along the path could not record the metric of
interest (either because of lack of knowledge or because this was
prevented by policy).

'C' flag. When set, this indicates that the Routing Metric/
Constraint object refers to a routing constraint. When cleared,
the routing object refers to a routing metric.

'O' flag: The 'O' flag is used exclusively for routing constraints
('C' flag is set). When set, this indicates that the constraint
specified in the body of the object is optional. When cleared,
the constraint is mandatory. If the 'C' flag is zero, the 'O'
flag MUST be set to zero on transmission and ignored on reception.

'R' flag: The 'R' flag is only relevant for a routing metric (C=0)
and MUST be cleared for C=1. When set, this indicates that the
routing metric is recorded along the path. Conversely, when
cleared, the routing metric is aggregated.

A Field (3 bits): The A field is only relevant for metrics and is
used to indicate whether the aggregated routing metric is additive,
is multiplicative, reports a maximum, or reports a minimum.

A=0: The routing metric is additive

A=1: The routing metric reports a maximum

A=2: The routing metric reports a minimum

A=3: The routing metric is multiplicative

The A field has no meaning when the 'C' flag is set (i.e., when the
Routing Metric/Constraint object refers to a routing constraint) and
is only valid when the 'R' bit is cleared. Otherwise, the A field
MUST be set to 0 and MUST be ignored on receipt.

Prec field (4 bits): The Prec field indicates the precedence of this
Routing Metric/Constraint object relative to other objects in the
container. This is useful when a DAG Metric Container contains
several Routing Metric objects. Its value ranges from 0 to 15. The
value 0 means the highest precedence.

Example 1: A DAG formed by RPL where all nodes must be mains-powered
and the best path is the one with lower aggregated expected
transmission count (ETX). In this case, the DAG Metric Container
carries two Routing Metric/Constraint objects: one is an ETX metric
object with header (C=0, O=0, A=00, R=0) and the second one is a Node
Energy constraint object with header (C=1, O=0, A=00, R=0). Note
that a RPL Instance may use the metric object to report a maximum
(A=1) or a minimum (A=2). If, for example, the best path is
characterized by the path avoiding low quality links, then the path
metric reports a maximum (A=1) (the higher the ETX, the lower the
link quality): when the DIO message reporting the link quality metric
(ETX) is processed by a node, each node selecting the advertising
node as a parent updates the value carried in the metric object by
replacing it with its local link ETX value if and only if the latter
is higher. As far as the constraint is concerned, the object body
will carry a Node Energy constraint object defined in Section 3.1
indicating that nodes must be mains-powered: if the constraint
signaled in the DIO message is not satisfied, the advertising node is
just not selected as a parent by the node that processes the DIO
message.

Example 2: A DAG formed by RPL where the link metric is the link
quality level (defined in Section 4) and link quality levels must be
recorded along the path. In this case, the DAG Metric Container
carries a Routing Metric/Constraint object: link quality level metric
(C=0, O=0, A=00, R=1) containing multiple sub-objects.

A Routing Metric/Constraint object may also include one or more
additional type-length-value (TLV) encoded data sets. Each Routing
Metric/Constraint TLV has the same structure:

Type: 1 byte
Length: 1 byte
Value: variable

A Routing Metric/Constraint TLV is comprised of 1 byte for the type,
1 byte specifying the TLV length, and a value field. The TLV length
field defines the length of the value field in bytes (from 0 to 255).

Unrecognized TLVs MUST be silently ignored while still being
propagated in DIOs generated by the receiving node.

IANA manages the codepoints for all TLVs carried in routing
constraint/metric objects.

IANA management of the Routing Metric/Constraint objects identifier
codespace is described in Section 6.

2.2. Use of Multiple DAG Metric Containers

Since the length of RPL options is encoded using 1 octet, they cannot
exceed 255 bytes, which also applies to the DAG Metric Container. In
the vast majority of cases, the advertised routing metrics and
constraints will not require that much space. However, there might
be circumstances where larger space is required, should, for example,
a set of routing metrics be recorded along a long path. In this
case, in order to avoid overflow, as specified in [RFC6550], routing
metrics will be carried using multiple DAG Metric Container objects.

In the rest of this document, this use of multiple DAG Metric
Container objects will be considered as if they were actually just
one long DAG Metric Container object.

2.3. Metric Usage

When the DAG Metric Container contains a single aggregated metric
(scalar value), the order relation to select the best path is
implicitly derived from the metric type. For example, lower is
better for Hop Count, Link Latency, and ETX. Conversely, for Node
Energy or Throughput, higher is better.

An example of using such a single aggregated metric is optimizing
routing for node energy. The Node Energy metric (E_E field) defined
in Section 3.2 is aggregated along paths with an explicit min
function (A field), and the best path is selected through an implied
Max function because the metric is Energy.

When the DAG Metric Container contains several aggregated metrics,
they are to be used as tiebreakers according to their precedence
defined by their Prec field values.

An example of such use of multiple aggregated metrics is the
following: Hop Count as the primary criterion, Link Quality Level
(LQL) as the secondary criterion, and Node Energy as the ultimate
tiebreaker. In such a case, the Hop Count, LQL, and Node Energy
metric objects' Prec fields should bear strictly increasing values
such as 0, 1, and 2, respectively.

If several aggregated metrics happen to bear the same Prec value, the
behavior is implementation dependent.

3. Node Metric/Constraint Objects

Sections 3 and 4 specify several link and node metric/constraint
objects. In some cases, it is stated that there must not be more
than one object of a specific type. In that case, if a RPL
implementation receives more than one object of that type, the second
object MUST silently be ignored.

In the presence of a constraint, a node MUST include a metric of the
same type. That metric is used to check whether or not the
constraint is met. In all cases, a node MUST not change the content
of the constraint.

3.1. Node State and Attribute Object

The Node State and Attribute (NSA) object is used to provide
information on node characteristics.

The NSA object MAY be present in the DAG Metric Container. There
MUST NOT be more than one NSA object as a constraint per DAG Metric
Container, and there MUST NOT be more than one NSA object as a metric
per DAG Metric Container.

The NSA object may also contain a set of TLVs used to convey various
node characteristics. No TLV is currently defined.

The NSA Routing Metric/Constraint Type has been assigned value 1 by
IANA.

Res flags (8 bits): Reserved field. This field MUST be set to zero
on transmission and MUST be ignored on receipt.

Flags field (8 bits). The following two bits of the NSA object are
currently defined:

'A' flag: data Aggregation Attribute. Data aggregation is listed
as a requirement in Section 6.2 of [RFC5548]. Some applications
may make use of the aggregation node attribute in their routing
decision so as to minimize the amount of traffic on the network,
thus, potentially increasing its lifetime in battery operated
environments. Applications where highly directional data flow is
expected on a regular basis may take advantage of data aggregation
supported routing. When set, this indicates that the node can act
as a traffic aggregator. Further documents MAY define optional
TLVs to describe the node traffic aggregator functionality.

'O' flag: node workload may be hard to determine and express in
some scalar form. However, node workload could be a useful metric
to consider during path calculation, in particular when queuing
delays must be minimized for highly sensitive traffic considering
Medium Access Control (MAC) layer delay. Node workload MAY be set
upon CPU overload, lack of memory, or any other node related
conditions. Using a simple 1-bit flag to characterize the node
workload provides a sufficient level of granularity, similar to
the "overload" bit used in routing protocols such as IS-IS.
Algorithms used to set the overload bit and to compute paths to
potentially avoid nodes with their overload bit set are outside
the scope of this document, but it is RECOMMENDED to avoid
frequent changes of this bit to avoid routing oscillations. When
set, this indicates that the node is overloaded and may not be
able to process traffic.

The unspecified flag fields MUST be set to zero on transmission and
MUST be ignored on receipt.

The Flags field of the NSA Routing Metric/Constraint object is
managed by IANA. Unassigned bits are considered as reserved.

3.2. Node Energy Object

It may sometimes be desirable to avoid selecting a node with low
residual energy as a router; thus, the support for constraint-based
routing is needed. In such cases, the routing protocol engine may
compute a longer path (constraint based) for some traffic in order to
increase the network life duration.

Power and energy are clearly critical resources in most LLNs. As
yet, there is no simple abstraction that adequately covers the broad
range of power sources and energy storage devices used in existing
LLN nodes. These include mains-powered, primary batteries, energy
scavengers, and a variety of secondary storage mechanisms.
Scavengers may provide a reliable low level of power, such as might
be available from a 4-20 mA loop; a reliable but periodic stream of
power, such as provided by a well-positioned solar cell; or
unpredictable power, such as might be provided by a vibrational
energy scavenger on an intermittently powered pump. Routes that are
viable when the sun is shining may disappear at night. A pump
turning on may connect two previously disconnected sections of a
network.

Storage systems, such as rechargeable batteries, often suffer
substantial degradation if regularly used to full discharge, leading
to different residual energy numbers for regular versus emergency
operation. A route for emergency traffic may have a different
optimum than one for regular reporting.

Batteries used in LLNs often degrade substantially if their average
current consumption exceeds a small fraction of the peak current that
they can deliver. It is not uncommon for self-supporting nodes to
have a combination of primary storage, energy scavenging, and
secondary storage, leading to three different values for acceptable
average current depending on the time frame being considered, e.g.,
milliseconds, seconds, and hours/years.

Raw power and energy values are meaningless without knowledge of the
energy cost of sending and receiving packets, and lifetime estimates
have no value without some higher-level constraint on the lifetime
required of a device. In some cases, the path that exhausts the
battery of a node on the bed table in a month may be preferable to a
route that reduces the lifetime of a node in the wall to a decade.

Given the complexity of trying to address such a broad collection of
constraints, this document defines two levels of fidelity in the
solution.

The simplest solution relies on a 2-bit field encoding three types of
power sources: "powered", "battery", and "scavenger". This simple
approach may be sufficient for many applications.

The mid-complexity solution is a single parameter that can be used to
encode the energetic happiness of both battery-powered and scavenging
nodes. For scavenging nodes, the 8-bit quantity is the power
provided by the scavenger divided by the power consumed by the
application, E_E=P_in/P_out, in units of percent. Nodes that are
scavenging more power than they are consuming will register above
100. A good time period for averaging power in this calculation may
be related to the discharge time of the energy storage device on the
node, but specifying this is out of the scope of this document. For
battery-powered devices, E_E is the current expected lifetime divided
by the desired minimum lifetime, in units of percent. The estimation
of remaining battery energy and actual power consumption can be
difficult, and the specifics of this calculation are out of scope of
this document, but two examples are presented. If the node can
measure its average power consumption, then E_E can be calculated as
the ratio of desired max power (initial energy E_0 divided by desired
lifetime T) to actual power, E_E=P_max/P_now. Alternatively, if the
energy in the battery E_bat can be estimated, and the total elapsed
lifetime, t, is available, then E_E can be calculated as the total
stored energy remaining versus the target energy remaining: E_E=
E_bat / [E_0 (T-t)/T].

An example of an optimized route is max(min(E_E)) for all battery-
operated nodes along the route, subject to the constraint that
E_E>=100 for all scavengers along the route.

Note that the estimated percentage of remaining energy indicated in
the E_E field may not be useful in the presence of nodes powered by
battery or energy scavengers when the amount of energy accumulated by
the device significantly differ. Indeed, X% of remaining energy on a
node that can store a large amount of energy cannot be easily
compared to the same percentage of remaining energy on a node powered
by a tiny source of energy. That being said, in networks where nodes
have similar energy storage, such a percentage of remaining energy is
useful.

The Node Energy (NE) object is used to provide information related to
node energy and may be used as a metric or as constraint.

The NE object MAY be present in the DAG Metric Container. There MUST
NOT be more than one NE object as a constraint per DAG Metric
Container, and there MUST NOT be more than one NE object as a metric
per DAG Metric Container.

The NE sub-object may also contain a set of TLVs used to convey
various nodes' characteristics.

Flags field (8 bits). The following flags are currently defined:

I (Included): the 'I' bit is only relevant when the node type is
used as a constraint. For example, the path must only traverse
mains-powered nodes. Conversely, battery-operated nodes must be
excluded. The 'I' bit is used to stipulate inclusion versus
exclusion. When set, this indicates that nodes of the type
specified in the node type field MUST be included. Conversely,
when cleared, this indicates that nodes of type specified in the
node type field MUST be excluded.

E (Estimation): when the 'E' bit is set for a metric, the
estimated percentage of remaining energy on the node is indicated
in the E_E 8-bit field. When cleared, the estimated percentage of
remaining energy is not provided. When the 'E' bit is set for a
constraint, the E_E field defines a threshold for the inclusion/
exclusion: if an inclusion, nodes with values higher than the
threshold are to be included; if an exclusion, nodes with values
lower than the threshold are to be excluded.

E_E (Estimated-Energy): 8-bit unsigned integer field indicating an
estimated percentage of remaining energy. The E_E field is only
relevant when the 'E' flag is set, and it MUST be set to 0 when the
'E' flag is cleared.

If the NE object comprises several sub-objects when used as a
constraint, each sub-object adds or subtracts node subsets as the
sub-objects are parsed in order. The initial set (full or empty) is
defined by the 'I' bit of the first sub-object: full if that 'I' bit
is an exclusion, empty if that 'I' bit is an inclusion.

No TLV is currently defined.

Future documents may define more complex solutions involving TLV
parameters representing energy storage, consumption, and generation
capabilities of the node, as well as desired lifetime.

3.3. Hop Count Object

The Hop Count (HP) object is used to report the number of traversed
nodes along the path.

The HP object MAY be present in the DAG Metric Container. There MUST
NOT be more than one HP object as a constraint per DAG Metric
Container, and there MUST NOT be more than one HP object as a metric
per DAG Metric Container.

The HP object may also contain a set of TLVs used to convey various
node characteristics. No TLV is currently defined.

Res flags (4 bits): Reserved field. This field MUST be set to zero
on transmission and MUST be ignored on receipt.

No Flag is currently defined. Unassigned bits are considered
reserved. They MUST be set to zero on transmission and MUST be
ignored on receipt.

The HP object may be used as a constraint or a metric. When used as
a constraint, the DAG root indicates the maximum number of hops that
a path may traverse. When that number is reached, no other node can
join that path. When used as a metric, each visited node simply
increments the Hop Count field.

Note that the first node along a path inserting a Hop Count metric
object MUST set the Hop Count field value to 1.

4. Link Metric/Constraint Objects

4.1. Throughput

Many LLNs support a wide range of throughputs. For some links, this
may be due to variable coding. For the deeply duty-cycled links
found in many LLNs, the variability comes as a result of trading
power consumption for bit rate. There are several MAC layer
protocols that allow for the effective bit rate of a link to vary
over more than three orders of magnitude with a corresponding change
in power consumption. For efficient operation, it may be desirable
for nodes to report the range of throughput that their links can
handle in addition to the currently available throughput.

The Throughput object MAY be present in the DAG Metric Container.
There MUST NOT be more than one Throughput object as a constraint per
DAG Metric Container, and there MUST NOT be more than one Throughput
object as a metric per DAG Metric Container.

The Throughput object is made of throughput sub-objects and MUST at
least comprise one Throughput sub-object. The first Throughput sub-
object MUST be the most recently estimated actual throughput. The
actual estimation of the throughput is outside the scope of this
document.

Throughput: 32 bits. The Throughput is encoded in 32 bits in
unsigned integer format, expressed in bytes per second.

4.2. Latency

Similar to throughput, the latency of many LLN MAC sub-layers can
vary over many orders of magnitude, again with a corresponding change
in power consumption. Some LLN MAC link layers will allow the
latency to be adjusted globally on the subnet, on a link-by-link
basis, or not at all. Some will insist that it be fixed for a given
link, but allow it to be variable from link to link.

The Latency object MAY be present in the DAG Metric Container. There
MUST NOT be more than one Latency object as a constraint per DAG
Metric Container, and there MUST NOT be more than one Latency object
as a metric per DAG Metric Container.

The Latency object is made of Latency sub-objects and MUST at least
comprise one Latency sub-object. Each Latency sub-object has a fixed
length of 4 bytes.

The Latency object may be used as a constraint or a path metric. For
example, one may want the latency not to exceed some value. In this
case, the Latency object common header indicates that the provided
value relates to a constraint. In another example, the Latency
object may be used as an aggregated additive metric where the value
is updated along the path to reflect the path latency.

4.3. Link Reliability

In LLNs, link reliability could be degraded for a number of reasons:
signal attenuation, interferences of various forms, etc. Time scales
vary from milliseconds to days, and are often periodic and linked to
human activity. Packet error rates can generally be measured
directly, and other metrics (e.g., bit error rate, mean time between
failures) are typically derived from that. Note that such
variability is not specific to wireless link but also applies to PLC
links.

A change in link quality can affect network connectivity; thus, link
quality may be taken into account as a critical routing metric.

A number of link reliability metrics could be defined reflecting
several reliability aspects. Two link reliability metrics are
defined in this document: the Link Quality Level (LQL) and the ETX
Metric.

Note that a RPL deployment MAY use the LQL, the ETX, or both.

4.3.1. The Link Quality Level Reliability Metric

The Link Quality Level (LQL) object is used to quantify the link
reliability using a discrete value, from 0 to 7, where 0 indicates
that the link quality level is unknown and 1 reports the highest link
quality level. The mechanisms and algorithms used to compute the LQL
are implementation specific and outside of the scope of this
document.

The LQL can be used either as a metric or a constraint. When used as
a metric, the LQL metric can only be recorded. For example, the DAG
Metric object may request all traversed nodes to record the LQL of
their incoming link into the LQL object. Each node can then use the
LQL record to select its parent based on some user defined rules
(e.g., something like "select the path with most links reporting a
LQL value of 3 or less").

Counters are used to compress the information: for each encountered
LQL value, only the number of matching links is reported.

The LQL object MAY be present in the DAG Metric Container. There
MUST NOT be more than one LQL object as a constraint per DAG Metric
Container, and there MUST NOT be more than one LQL object as a metric
per DAG Metric Container.

The LQL object MUST contain one or more sub-object used to report the
number of links along with their LQL.

4.3.2. The ETX Reliability Object

The ETX metric is the number of transmissions a node expects to make
to a destination in order to successfully deliver a packet. In
contrast with the LQL routing metric, the ETX provides a discrete
value (which may not be an integer) computed according to a specific
formula: for example, an implementation may use the following
formula: ETX= 1 / (Df * Dr) where Df is the measured probability that
a packet is received by the neighbor and Dr is the measured
probability that the acknowledgment packet is successfully received.
This document does not mandate the use of a specific formula to
compute the ETX value.

The ETX object MAY be present in the DAG Metric Container. There
MUST NOT be more than one ETX object as a constraint per DAG Metric
Container, and there MUST NOT be more than one ETX object as a metric
per DAG Metric Container.

The ETX object is made of ETX sub-objects and MUST at least comprise
one ETX sub-object. Each ETX sub-object has a fixed length of 16
bits.

ETX: 16 bits. The ETX * 128 is encoded using 16 bits in unsigned
integer format, rounded off to the nearest whole number. For
example, if ETX = 3.569, the object value will be 457. If ETX >
511.9921875, the object value will be the maximum, which is 65535.

The ETX object may be used as a constraint or a path metric. For
example, it may be required that the ETX must not exceed some
specified value. In this case, the ETX object common header
indicates that the value relates to a constraint. In another
example, the ETX object may be used as an aggregated additive metric
where the value is updated along the path to reflect the path
quality: when a node receives the aggregated additive ETX value of
the path (cumulative path ETX calculated as the sum of the link ETX
of all of the traversed links from the advertising node to the DAG
root), if it selects that node as its preferred parent, the node
updates the path ETX by adding the ETX of the local link between
itself and the preferred parent to the received path cost (path ETX)
before potentially advertising itself the new path ETX.

4.4. Link Color Object

4.4.1. Link Color Object Description

The Link Color (LC) object is an administrative 10-bit link
constraint (which may be either static or dynamically adjusted) used
to avoid or attract specific links for specific traffic types.

The LC object can be used either as a metric or as a constraint.
When used as a metric, the LC metric can only be recorded. For
example, the DAG may require recording the link colors for all
traversed links. A color is defined as a specific set of bit values:
in other words, that 10-bit field is a flag field, and not a scalar
value. Each node can then use the LC to select the parent based on
user defined rules (e.g., "select the path with the maximum number of
links having their first bit set 1 (e.g., encrypted links)"). The LC
object may also be used as a constraint.

When used as a recorded metric, a counter is used to compress the
information where the number of links for each Link Color is
reported.

The Link Color (LC) object MAY be present in the DAG Metric
Container. There MUST NOT be more than one LC object as a constraint
per DAG Metric Container, and there MUST NOT be more than one LC
object as a metric per DAG Metric Container.

Reserved (5 bits): Reserved field. This field MUST be set to zero on
transmission and MUST be ignored on receipt.

'I' Bit: The 'I' bit is only relevant when the Link Color is used as
a constraint. When set, this indicates that links with the specified
color must be included. When cleared, this indicates that links with
the specified color must be excluded.

It is left to the implementer to define the meaning of each bit of
the 10-bit Link Color Flag field.

4.4.2. Mode of Operation

The link color may be used as a constraint or a metric.

When used as constraint, the LC object may be inserted in the DAG
Metric Container to indicate that links with a specific color
should be included or excluded from the computed path.

When used as recorded metric, each node along the path may insert
an LC object in the DAG Metric Container to report the color of
the local link. If there is already an LC object reporting a
similar color, the node MUST NOT add another identical LC sub-
object and MUST increment the counter field.

5. Computation of Dynamic Metrics and Attributes

As already pointed out, dynamically calculated metrics are of the
utmost importance in many circumstances in LLNs. This is mainly
because a variety of metrics change on a frequent basis, thus,
implying the need to adapt the routing decisions. That being said,
care must be given to the pace at which changes are reported in the
network. The attributes will change according to their own time
scales. RPL controls the reporting rate.

To minimize metric updates, multi-threshold algorithms MAY be used to
determine when updates should be sent. When practical, low-pass
filtering and/or hysteresis should be used to avoid rapid
fluctuations of these values. Finally, although the specification of
path computation algorithms using dynamic metrics is out of the scope
of this document, it is RECOMMENDED to carefully design the route
optimization algorithm to avoid too frequent computation of new
routes upon metric values changes.

Controlled adaptation of the routing metrics and rate at which paths
are computed are critical to avoid undesirable routing instabilities
resulting in increased latencies and packet loss because of temporary
micro-loops. Furthermore, excessive route changes will adversely
impact the traffic and power consumption in the network, thus,
potentially impacting its scalability.

6. IANA Considerations

IANA has established a new top-level registry, called "RPL Routing
Metric/Constraint", to contain all Routing Metric/Constraint objects
codepoints and sub-registries.

The allocation policy for each new registry is by IETF review: new
values are assigned through the IETF review process (see [RFC5226]).
Specifically, new assignments are made via RFCs approved by the IESG.
Typically, the IESG will seek input on prospective assignments from
appropriate persons (e.g., a relevant working group if one exists).

New bit numbers may be allocated only by an IETF Review action. Each
bit should be tracked with the following qualities:

Bit number

Capability Description

Defining RFC

6.1. Routing Metric/Constraint Type

IANA has created a sub-registry, called "Routing Metric/Constraint
Type", for Routing Metric/Constraint object types, which range from 0
to 255. Value 0 is unassigned.

6.2. Routing Metric/Constraint TLVs

IANA has created a sub-registry, called "Routing Metric/Constraint
TLVs", used for all TLVs carried within Routing Metric/Constraint
objects. The Type field is an 8-bit field whose value is comprised
between 0 and 255. Value 0 is unassigned. The Length field is an
8-bit field whose value ranges from 0 to 255. The Value field has
value ranges depending on the Type; therefore, they are not defined
here, since no Type is registered at this time.

6.3. Routing Metric/Constraint Common Header Flag Field

IANA has created a sub-registry, called "Routing Metric/Constraint
Common Header Flag field", to manage the 9-bit Flag field of the
Routing Metric/Constraint common header.

Several bits are defined for the Routing Metric/Constraint common
header Flag field in this document. The following values have been
assigned:

6.6. Hop-Count Object Flags Field

IANA has created a sub-registry, called "Hop-Count Object Flag
field", to manage the codespace of the 4-bit Flag field of the Hop
Count object.

No Flag is currently defined.

6.7. Node Type Field

IANA has created a sub-registry, called "Node Type Field", to manage
the codespace of the field of the Routing Metric/Constraint common
header.

The T field is 2 bits in length, and it has values ranging from 0 to
3.

Codespace of the T field (Routing Metric/Constraint common header)

Value Description Reference
0 a mains-powered node This document
1 a battery-powered node This document
2 a node powered by an energy scavenger This document

7. Security Considerations

Routing metrics should be handled in a secure and trustful manner.
For instance, RPL should not allow a malicious node to falsely
advertise that it has good metrics for routing so as to be selected
as preferred next-hop router for other nodes' traffic and intercept
packets. Another attack may consist of making intermittent attacks
on a link in an attempt to constantly modify the link quality and
consequently the associated routing metric, thus, leading to
potential fluctuation in the DODAG. Thus, it is RECOMMENDED for a
RPL implementation to put in place mechanisms so as to stop
advertising routing metrics for highly unstable links that may be
subject to attacks.

Some routing metrics may also be used to identify some areas of
weaknesses in the network (a highly unreliable link, a node running
low in terms of energy, etc.). Such information may be used by a
potential attacker. Thus, it is RECOMMENDED to carefully consider
which metrics should be used by RPL and the level of visibility that
they provide about the network state or to use appropriate the
security measures as specified in [RFC6550] to protect that
information.

Since the routing metrics/constraints are carried within RPL message,
the security routing mechanisms defined in [RFC6550] apply here.